.box {
  background-color: #ddd;
}
.box .back {
  display: none;
  position: fixed;
  bottom: 30px;
  right: 30px;
  width: 48px;
  background-color: rgba(0, 0, 0, 0.5);
  padding: 5px 8px;
  border-radius: 3px;
  color: #ffffff;
  cursor: pointer;
}
.box .back:hover {
  background-color: rgba(0, 0, 0, 0.7);
}
